The present invention relates to an improvement to transesterification processes for vegetable or animal oils. It more specifically relates to the use of generally lost or low value oily phases which, after transformation, can be recycled and therefore added to the oil used as the starting product for the production of esters.
The integration of said recycling into conventional processes makes it possible to obtain with a quasi-quantitative yield compared with the oil used, methyl, ethyl, propyl or butyl esters used as bases for lubricants, but also as substitute fuels for gas oil. This surprising integration in no way reduces the quality of the esters obtained.
In theory and in practice, it is known how it is possible to produce esters of fatty substances by reacting a neutral oil and a low molecular weight dry alcohol. The ester molar yield is approximately 97 to 98%. The problem is more difficult in the case of the starting material being an acid oil and/or a hydrated alcohol. The state of the art offers numerous solutions, but none seems to be economically and technically satisfactory if consideration is given to the yields obtained.
The first difficulties are encountered with acid oils. It has been proposed that the acidity be neutralized and, following decanting or centrifuging, reacting the neutralized oil with a dry alcohol in the presence of alkaline catalysts. This is in fact the standard procedure used at present in the oil industry. However, there is not only a loss of the fatty acid, but also an oil fraction entrained by the soaps in the aqueous phase, which fraction can be equivalent in weight to that of the fatty acids. This loss is particularly high with certain palm or coconut oils, for which the acidity sometimes rises up to 10% by weight of acid.
Another way to treat an acid oil for bringing it to a state where it can easily be transformed into ester is to reduce the acidity by physical refining, which is less expensive with regards to entrained products than a neutralization, but is more expensive as regards energy and capital expenditure. Once again the acid weight contained in the oil is lost.
With very acid oils, consideration can be given to the esterification thereof with a low molecular weight alcohol in the presence of an acid catalyst. The catalyst is either a soluble acid, or an ion exchange resin. However, in order to obtain an oil with only slight acidity, it is also necessary to eliminate the condensation water resulting from the action of the alcohol on the fatty acid. To eliminate said water, use is made either of an external dryer which must consequently be regenerated, or a system for the entrainment of the water by the alcohol. This gives rise to the hydration of the alcohol, which can no longer be easily reused, because the traces of water catalyze the saponification of the esters if the oils are esterified with a basic catalyst in the following stage. If this alcohol is used for carrying out the transesterification stage, by no means negligible quantities of the soaps formed are lost.
Another case which is relatively frequently encountered in tropical countries, where the alcohol which is to be used in the transesterification is hydrated, such as is e.g. the case with 96 vol % ethyl alcohol. Here again a procedure has been proposed, which consists of using acid catalysts according to two variants.
The first variant, more particularly described in U.S. Pat. No. 4,652,406simply brings the oil to the neutral state by the esterification of the free acids. However, this esterification increases in difficulty with the alcohol-supplied water present. Competition takes place between the alcohol and the water and it is not possible to sufficiently reduce the acidity except by using an external dryer or a third solvent.
Although this system leads to a reduction of the losses resulting from the acidity, it is necessary to further reduce those occurring on transesterifying the oil with a basic catalyst. Thus, as generally approximately 90% of the oil has to be converted, the situation is that of a neutral oil which is to be transesterified with a hydrated alcohol. The soap losses are significant.
The second variant, more particularly described in U.S. Pat. No. 4,695,411 consists of carrying out an acid catalysis under more severe conditions, where 80% of the oil is converted into esters. The glycerol formed and decanted entrains most of the water. The ester phase, which is incompletely converted, contains water, alcohol and a large amount of fatty acid. It is possible to carry out esterification by drying the alcohol to reduce the acidity. There is finally a total conversion in the presence of a basic catalyst and a hydrated or non-hydrated alcohol. The difference compared with the previous process where only the fatty acids are esterified, is that only approximately 10 to 15% of the oil remains for conversion into esters. However, the soap losses can be high, e.g. 5 to 8%, when a hydrated alcohol is used.
In the latter case, it would be possible to neutralize the fatty acids present in the ester phase instead of esterifying and also to decant the same, which would avoid the use of an external dryer. However, once again a by no means negligible fraction of the soaps and entrained esters is lost.
Finally, reference is made to a case where use is made of a hydrated alcohol and a basic catalyst in the presence of an acid or non-acid oil. The losses are considerable because the basic catalyst rapidly saponifies the oil. Therefore the soaps are not catalysts at conventional temperatures. Thus, e.g. with a 95 vol. % ethyl alcohol, dry colza oil and 1.5% soda in powder form based on the oil, the ester yield is 83% by weight, whereas theoretically and without loss it would be 105% by weight and 94% at the stoichiometry of the soda, excepting that all the soda is transformed into soap.
Thus, no prior art process would appear to be satisfactory, even those using dry alcohol and neutralized oil, because again there is a 2 to 3% loss. All the other processes lead to even greater losses compared with the theoretical ester yield and they can reach 30% by weight based on the oil.